Urban Conflagrations: Structural Ash and Soil Metal(loid) Contamination after California’s Eaton and Palisades Fires

In the weeks after California’s Eaton-Palisades wildfires, structural ash of burned homes and soils from yards and curbsides were sampled for analysis of Pb, As, and eight metals from 32 burned residences. About six months after fires and after US Army Corps of Engineers’ Phase 2 cleanups removed 15 cm from structural ash footprints, 17 properties were resampled within about a meter of original sampling sites. Concentrations of metal(oid)s in structural ash and residential soils varied between and within properties with significant spatial and temporal patterns. Lead concentrations were highest in structural ash and soil of yards of select pre-1970s homes, the yard and curbside soils likely having Pb contaminants from before fires. Approximately 28% of structural-ash samples exceeded 200 ppm Pb, the residential soil screening level (SSL) of the US Environmental Protection Agency, and 50% exceeded 80 ppm, California’s Pb SSL. After cleanup, < 5% of samples from within the scraped foundation areas exceeded 200 ppm Pb and 24% exceeded 80 ppm. Interpreting postcleanup risk from Pb and As depends greatly on the California or federal SSLs. Fires released metals from select older homes to residential soils in concentrations comparable to those found in unburned city soils.